For the flow measurement of flowable media (e.g. fluids) in pipelines, especially of gas flows or vapor flows at high temperature ranges, frequently vortex, flow measuring devices are applied. In the case of such vortex, flow measuring devices, a bluff body, which forms a flow obstruction for the flowing medium, is arranged in such a manner in the flow path that the medium can flow past on both sides of the flow obstruction. In such case, vortices are shed on both sides of the flow obstruction. Over a wide range of Reynolds numbers, the vortices are shed, in such case, alternately on the two sides of the flow obstruction, so that a staggered arrangement of vortices arises. This staggered arrangement of vortices is referred to as a Kármán vortex street. In the case of vortex, flow measuring devices, the principle is utilized that the vortex shedding frequency, with which these vortices are formed, is proportional to the flow velocity, respectively to the volume flow, of the respective medium over a wide range of Reynolds numbers. The produced vortices are registered by sensor in the vortex, flow measuring device. Accordingly, from the registered vortex shedding frequency of the vortices (in the following referred to as the vortex frequency) and a calibration factor characteristic for the given type of vortex, flow measuring device, the flow velocity, respectively the volume flow, can be determined.
If process conditions within the pipeline lie in the region of a phase transformation of the flowing medium or of a material contained in the medium, then a phase transformation can take place in the medium. The medium then exists as two or more phases. Especially, a part of a gaseous medium can condense out. In the case of determining the volume flow or mass flow by a vortex, flow measuring device, there is, in such case, the problem that the device has within the measuring tube in the region of the bluff body (the measuring cross section) a reduced flow cross section compared with sections of the measuring tube upstream and downstream from the bluff body. In this way, the flow velocity of the medium in the region of the measuring cross section is locally increased. In the case of incompressible media, this effect can be relatively simply included in a calibration factor, by which the ratio (as established by the ratio of the flow cross sections) between the flow velocity in the region of the measuring cross section and the flow velocity in a cross section (subsequently referred to herein as the connecting pipe region) located upstream or downstream of the measuring cross section is taken into consideration. In the case of compressible media, such as, for example, in the case of gases, this ratio is, however, not fixed. For example, there occur in the case of gaseous media in the region of the measuring cross section a significant temperature decrease and a pressure drop. The density of the medium differs in the region of the measuring cross section significantly from the density in the connecting pipe region. Additionally, the case can occur, in which, due to the different process conditions in the region of the measuring cross section, a part of the medium condenses, while in the connecting pipe region, it evaporates again. This leads to a considerable deviation of the density of the medium in the region of the measuring cross section from the density of the medium in the connecting pipe region. In general, the problem can occur, in which, due to the process conditions present in the region of the measuring cross section, a part of the medium undergoes a phase transformation, and this phase transformation is subsequently reversed in the connecting pipe region. If the flow velocity in the connecting pipe region is calculated with a constant calibration factor, which is specifically for the respective type of vortex, flow measuring device, starting from the flow velocity measured locally in the region of the measuring cross section (alternatively the volume flow measured locally in the region of the measuring cross section), then an error arises due to the above explained effects. This problem exists especially when a phase transformation of a part of the medium occurs in the region of the measuring cross section due to the process conditions arising locally in the region of the measuring cross section.